Basic Concepts of Groundwater Hydrology

1 In partnership Farm water Quality PlanningA water Quality and Technical Assistance Program for california REFERENCE SHEET is part ofthe Farm water QualityPlanning (FWQP) series,developed for a short coursethat provides training for grow-ers of irrigated crops who areinterested in implementingwater quality protection prac-tices. The short course teachesthe Basic Concepts of water -sheds, nonpoint source pollution(NPS), self-assessment tech-niques, and evaluation tech-niques. Management goals andpractices are presented for avariety of cropping : Basic Concepts of GroundwaterHydrologyTHOMAS HARTER is UC Cooperative Extension Hydrogeology Specialist, University ofCalifornia, Davis, and Kearney Agricultural depends on water .

2 Our entire living world plants, animals, and humans isunthinkable without abundant water . Human cultures and societies have ralliedaround water resources for tens of thousands of years for drinking, for food produc-tion, for transportation, and for recreation, as well as for , more than a third of all water used by humans comes from Groundwater . In rural areas the percentage is even higher: more than half of all drinkingwater worldwide is supplied from ground california , rural areas dependence on ground water is even greater. Californiahas 8,700 public water supply systems. Of these, 7,800 rely on ground water , drawingfrom more than 15,000 wells.

3 In addition, there are tens of thousands of privatelyowned wells used for domestic water supply within the state. Although sufficientaquifers for this sort of use underlie much of california (Figure 1), the large metro-politan areas in Southern california and the San Francisco Bay Area rely primarily onsurface water for their drinking water supplies. Overall, ground water supplies one-third of the water used in california in a typical year, in drought years as much IS ground water ?Despite our heavy reliance on ground water , its nature remains a mystery to manypeople. Many find it hard to imagine that water can move underground at rates suffi-cient to allow california s largest springs to discharge almost 1 million gallons perminute ( , Fall River in Shasta County).

4 Likewise, it is hard to understand how adomestic or irrigation well can extract from 500 to 2,000 gallons of water per minuteout of a pipe in the ground that is merely 1 foot in diameter. More often than not,people envision that ground water exists somehow in a mysterious, hidden system ofunderground rivers, reservoirs, and water veins. Although these terms may be use-ful when speaking metaphorically about ground water , they are far from water is water that fills pores and fractures in the ground , much as milkfills the voids within bits of granola in a breakfast bowl (Figure 2). The top of Groundwater is called the water table.

5 Between the water table and the land surface is theunsaturated zone or vadose zone. In the unsaturated zone, moisture is moving down-ward to the water table to recharge the ground water . The water table can be veryclose to the surface (within a few feet), or very deep (up to several hundred feet). Inmost california regions, the water table is between 10 and 100 feet below the landsurface (in some Southern california desert basins it is as deep as 300 feet).It is in california s numerous valleys and intermontane basins that ground waterexists in the greatest quantity ( california and Nevada Groundwater basins are shownin Figure 1).

6 The basins are like large bathtubs enclosed by the rocks of surroundingmountains. Over millions of years, these bathtubs were filled with hundreds andeven thousands of feet of sediment and debris that were carried into the basins byPUBLICATION 8083 FWQP REFERENCE SHEET OF CALIFORNIAD ivision of Agriculture and Natural and floods (Figure 3). In these so-called alluvial basins, ground water fillssmall, often microscopic pores between thegrains of gravel, sand, silt, and the rocks that make up the hills andmountains of california s uplands, Groundwater is also quite common, although inmost cases not nearly as plentiful.

7 In theserocks ground water occupies practicallyevery fracture and fissure below the watertable. However, unless fractures are largeand numerous, little water can be , AQUITARDS, ANDSPRINGSA geologic formation from which significantamounts of ground water can be pumpedfor domestic, municipal, or agricultural usesis known as an aquifer. In some cases,aquifers are vertically separated from eachother by geologic formations that permit lit-tle or no water to flow in or out. A forma-tion that acts as such a water barrier iscalled aquitard if it is much less permeablethan a nearby aquifer but still permits flow( , sandy clay).

8 If the water barrier isalmost impermeable ( , clay) and forms aformidable flow barrier between aquifers, itis known as an can be of two major types:unconfined or confined. An unconfinedaquifer has no overlying aquitard oraquiclude (Figure 4). Where there are multiple levels of aquifers, the uppermostaquifer typically is unconfined. Vertical recharge of an unconfined aquifer by rainwa-ter or irrigation water that filters downward through the soil is not restricted. Thewater table at the top of the unconfinedaquifer can migrate freely up and down with-in the sediment formation, depending onhow much water is stored there (Figure 3).

9 The water level in a borehole drilled into anunconfined aquifer will be at the same depthas the water table in the aquifer. A confined aquifer, on the other hand,is sandwiched between an aquitard above andan aquiclude or aquitard ( , bedrock)below (Figure 4). Because the water table inthe recharge area of the confined aquifer ismuch higher than the top of the confinedaquifer itself, water in a confined aquifer ispressurized. This pressurization means that2 ANR Publication 8083well-sorted sedimentpoorly sorted sedimentporous sedimentconsolidated sedimentdissolution of rockrock fracturesFigure 1. Groundwater basins of california and Nevada (Source: ,USGS Groundwater Atlas Figure 11 in section HA 730-B).

10 Figure 2. Types of pore space in sediments and and Range aquifersBasin fill aquifersCarbonate-rock aquifersVolcanic rock aquifersCentral Valley aquifer systemCoastal Basins aquifersMiocene basaltic-rock aquifersNorthern california basin-fill aquifersNorthern california volcanic-rock aquifersNot a principal aquiferVolcanic- and sedimentary-rock aquifersBase modified from Geological Survey digital data,1:2,000,000, 1972 Albers Equal-Area Conic projection Standard parallels 29 30 and 45 30 , central meridian -96 00 0255075100 miles0255075100 KILOMETERSthe water level in a borehole drilled into aconfined aquifer will rise significantly abovethe top of the aquifer.